There are no filler episodes
I actually think “filler” does tend to have an inherent negative connotation. Here’s how I’d describe it:
Each season in Lexx tends to have an over-arcing story, during which there are other self-contained stories, or subplots that span several episodes.
Season 4 is almost an exception to this, though. It has such an ecclectic and diverse number of themes and plots, it’s difficult to label one of them as the overall seasonal story-arc. For this season, I can’t be completely sure what the main story-arc is until the last episode airs.
I would define “filler” as an episode which:
A. Has little or nothing to do with the seasonal story-arc.
AND
B. By default, is really not all that good.
For example, if you were going to try and show the Lexx series as a whole to a friend of yours, using as few episodes as you possibly can (say, you have limited time), the episodes you leave out would tend to be “filler.”
Now, it IS possible to have GOOD “filler” episodes. But you generally have to make special mention of the fact that the episode is still good. The gut reaction when hearing that an episode is “filler,” is that it’s an episode not worth watching.
So, there are good filler episodes, but as a general rule, “filler” episodes are not that great.
An example of (bad) Filler:
Mort
An example of good Filler:
Wake the Dead
Alternatively, “key” episodes are episodes that do deal with the over-arcing seasonal story. These episodes are, by default, good. But at the same time, there are such things as BAD key episodes.
An example of a (good) key episode:
Norb
An example of a bad key episode:
White Trash
That’s my take.